Apparatus with spread-pulse modulation and nonlinear time domain equalization for fiber optic communication channels

ABSTRACT

A transmitter and transceiver for an optical communication channel may include a data encoder, a partial response (PR) precoder, a pulse filter, and an electrical-to-optical converter to transmit a spread pulse signal over an optical communication channel. The data encoder to encode the transmit data into coded data. The precoder to correlate bits of the coded data together into a precoded signal at the output of the precoder. The pulse filter to spread out the pulses in the precoded signal into a spread-pulse signal at the output of the pulse-shaping filter. The electrical-to-optical converter to convert an electrical spread-pulse signal into light pulses at its optical output. A transceiver may further include an optical-to-electrical converter, an automatic gain controller, a matched filter, a PR finite impulse response equalizing filter, a maximum likelihood sequence estimation detector, and a data decoder in order to decode and recover the data.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application and claims the benefit ofU.S. patent application Ser. No. 11/117,228, filed by Salam Elahmadi etal. on Apr. 28, 2005, now pending.”

FIELD

Embodiments of the invention generally relate to optical data linksincluding wavelength division multiplexing (WDM) fiber optictransmitters, receivers and transceivers. Particularly, embodiments ofthe invention relate to modulating, encoding, and decoding data forcommunication over a fiber optic cable and other dispersive media.

GENERAL BACKGROUND

In order to lower the cost of communication, it has become desirable toincrease the data rate and the number of communication channelsavailable. This is particularly true in fiber optic communicationsystems.

In fiber optic communication systems, wavelength division multiplexing(WDM) has been used over the same fiber optic communication link so thatmultiple channels of communication may be established over one fiberoptic cable. The multiple channels of communication are established atdifferent center wavelengths of light. However, the complexity of WDMand its higher data rates makes it expensive to use in low costapplications.

In the data link between fiber optic transceivers, emphasis has beenplaced on improving the electro-optic elements (EOE) and the opticalelements (OE) in order to provide for the increased data rates over thefiber optic cables. For example, the laser driver driving asemiconductor laser has been improved in order to maintain a wide dataeye from transmitter to receiver and avoid data bit errors at high datarates. While these improvements have marginally increased the data rate,they have not alleviated the need for high capacity optical links withlower cost and simpler operation.

Additionally, the medium of the fiber optic cable used has beencompensated for various optical signal impairments in order toaccommodate higher data rates and reduce some types of distortion.However, current compensation techniques operating in the optical domainare bulky, expensive, and consume too much power. Moreover, they onlycompensate for one type of distortion at a time, such as chromaticdispersion, and ignore other types of distortions. Furthermore, addingoptical signal distortion compensators along an optical cable rendersthe network provisioning process more complicated and significantlyincreases the network operational expenses. Additionally, replacingexisting lower data rate engineered fiber optic cables with compensatedcables to lower distortion and to support higher data rates is veryexpensive.

The need for improved, cost-efficient distortion-mitigating techniquesis important to lower the cost of today's optical communicationsnetworks, enhance their performance, streamline and simplify theirdeployment and operation.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the invention will becomeapparent from the following detailed description in which:

FIG. 1A is an exemplary block diagram of a first fiber opticcommunication system.

FIG. 1B is an exemplary block diagram of a fiber optic transceivermodule.

FIG. 1C is an exemplary block diagram of a second fiber opticcommunication system.

FIG. 2 is a high level block diagram of the electrical elements withinfiber optic transceiver modules of an fiber optic communication system.

FIG. 3A is a functional block diagram of the electrical elements forcommunication over a fiber optic link between fiber optic transceivermodules of an fiber optic communication system.

FIG. 3B is a flow chart corresponding to transmission of data over thefiber optic link by the functional blocks of FIG. 3A.

FIG. 3C is a flow chart corresponding to reception of data from thefiber optic link by the functional blocks of FIG. 3A.

FIG. 4 is a block diagram of an adaptive finite impulse response (FIR)filter as one embodiment of the partial response filter.

FIG. 5A is a functional block diagram of a partial response signalencoding of a second order with a data input of zero and one.

FIG. 5B is a multi state trellis state diagram in accordance with thesecond order partial response signal encoding of FIG. 5A.

FIG. 6A is a multi state trellis state diagram for the second orderpartial response signal encoding of FIG. 5A with general data inputsymbols of positive a (+a) and negative a (−a).

FIG. 6B illustrate equations of a metric update algorithm for the multistate trellis state diagram of FIG. 6A.

FIG. 6C illustrate a chart of the conditions used to implement theequations of the metric update algorithm illustrated in FIG. 6B for themulti state trellis state diagram of FIG. 6A.

FIG. 7A is a first functional block diagram of elements within a fiberoptic transceiver module.

FIG. 7B is a second functional block diagram of elements within a fiberoptic transceiver module.

FIG. 8 is a perspective view of an exemplary fiber optical transceivermodule including embodiments of the invention.

FIG. 9A is a waveform diagram of first simulation results illustratingtransmit and receive signals.

FIG. 9B is a waveform diagram of second simulation results illustratingtransmit and receive signals.

DETAILED DESCRIPTION

Embodiments of the invention set forth in the following detaileddescription generally relate to methods, apparatus, software, andsystems for mitigating the distortions, both linear and nonlinear, thataffect light pulses as they propagate in an optical fiber medium.

The embodiments of the invention use a new modulation and equalizationmethod that operates in the time-domain to compensate a signal fororders of chromatic and polarization mode dispersive effects, whichcause broadening of light pulses in an optical fiber, and combatnonlinear effects such as Raman scattering and Self Phase Modulation,and Cross Phase Modulation, in order to restore the shape of the opticalpulses at a receiver.

The embodiments of the invention are summarized by the claims. A methodfor an optical communication channel is provided by preconditioning adata signal prior to transmission over a fiber optic cable to minimizesignal distortion; converting the data signal into an optical signal andcoupling the optical signal into a first end of the fiber optic cable;receiving the optical signal from a second end of the fiber optic cableopposite the first end and converting the optical signal into anelectrical signal; and recovering the data signal from the electricalsignal. The preconditioning of the data signal prior to transmission mayinclude correlating bits of the data signal to minimize errorpropagation at a receiver and spreading out the pulses in the datasignal to avoid distortion over the optical communication channel. Thepreconditioning of the data signal prior to transmission may furtherinclude encoding the data signal using a run length limited code toexclude undesired patterns and aid clock recovery at the receiver. Therecovering of the data signal from the electrical signal may includefiltering the electrical signal to optimize a signal to noise ratio,shaping the spectrum of the received electrical signal, and removingintersymbol interference (ISI) from the electrical signal. Therecovering of the data signal from the electrical signal may furtherinclude maintaining an amplitude of the electrical signal over a rangeof predetermined amplitudes.

A method for an optical communication channel is provided by encodingdata into coded data using a run length limited code; correlating thecoded data into a precoded signal to minimize error propagation at areceiver; spreading out the pulses in the precoded signal into aspread-pulse signal to avoid distortion over the optical communicationchannel; and transmitting the spread-pulse signal over the opticalcommunication channel. The spread-pulse signal may be transmitted aslight pulses over the fiber optic cable of the optical communicationchannel. The transmitting may include converting the spread-pulse signalfrom an electrical signal into an optical spread-pulse signal, andcoupling the optical spread-pulse signal into a fiber optic cable totransmit the spread-pulse signal over the optical communication channel.The data may be encoded into coded data by a run length limited encoderusing the run length limited code, the coded data may be correlated intothe precoded signal by a precoder, and the pulses in the precoded signalmay be spread out into a spread-pulse signal using a pulse filter. Themethod for the optical communication channel may be further provided byreceiving the spread-pulse signal from the optical communicationchannel; filtering the spread-pulse signal to optimize a signal to noiseratio; shaping the spread-pulse signal into an equalized partialresponse signal to equalize linear distortions; removing the remainingintersymbol interference (ISI) from the equalized partial responsesignal; and decoding the equalized partial response signal to generatereceived data using the run length limited code. Prior to filtering thespread-pulse signal, the method for the optical communication channelmay be further provided by maintaining an amplitude of the spread-pulsesignal within a predetermined range of amplitudes. The receiving mayinclude decoupling an optical signal from the fiber optic cable toreceive the spread-pulse signal over the optical communication channel;and converting the spread-pulse signal from an optical signal into anelectrical signal.

Another method for an optical communication channel is provided byreceiving an optical spread-pulse signal from a first fiber optic cableof the optical communication system at a first receiver; converting theoptical spread-pulse signal into an electrical spread-pulse signal;filtering the electrical spread-pulse signal to optimize a signal tonoise ratio; shaping the electrical spread-pulse signal into anequalized partial response signal; removing the remaining intersymbolinterference (ISI) from the equalized partial response signal; anddecoding the equalized partial response signal to generate receiveddata. Prior to filtering the received electrical spread-pulse signal,the method for the optical communication channel may be further providedby maintaining an amplitude of the electrical spread-pulse signal withina predetermined range of amplitudes. The amplitude of the electricalspread-pulse signal may be maintained using an automatic gaincontroller. The electrical spread-pulse signal may be filtered using amatched filter. The electrical spread-pulse signal may be shaped intothe equalized partial response signal using a partial response filter.The intersymbol interference (ISI) may be removed from the equalizedpartial response signal using a maximum likelihood sequence estimation(MLSE) detector. An optical-to-electrical converter may convert theoptical spread-pulse signal into the electrical spread-pulse signal. Therecovered data from the MLSE may be further decoded by a run lengthlimited decoder using a run length limited code that was used to encodethe data prior to receiving. The method for the optical communicationchannel may be further provided by encoding transmit data into codeddata using a code; correlating the coded data into a precoded signal tominimize error propagation at a second receiver; spreading out thepulses in the precoded signal into a spread-pulse transmit signal;converting the spread-pulse transmit signal into an optical spread-pulsetransmit signal; and coupling the optical spread-pulse transmit signalinto a second fiber optic cable of the optical communication system. Thetransmit data may be encoded into coded data by a run length limitedencoder. The coded data may be correlated into the precoded signal by aprecoder. The pulses in the precoded signal may be spread out into aspread-pulse signal using a pulse filter. The spread-pulse signal may beconverted into the optical spread-pulse signal and coupled into thefiber optic cable by an electrical-to-optical converter.

Referring now to FIG. 1A, a first exemplary fiber optic communicationsystem 100 is shown. In the fiber optic communication system 100, afirst host system 101A is optically coupled to a second host system 101Bby means of the optical communication channels 102A-102N. Each opticalcommunication channel 102A-102N may be bi-directional and include afirst fiber optic communication link 104 and a second fiber opticcommunication link 106. If unidirectional communication is only desired,one of the first or second fiber optic communication links 104,106 cansuffice for the communication channel depending upon the direction ofdata transfer desired. Each fiber optic communication link 104,106represents a fiber optic cable.

Wavelength division multiplexing (WDM) may be used over the each fiberoptic communication link to accommodate multiple channels ofcommunication over one fiber optic cable. Bi-directional communicationmay also be provided over one fiber optic communication link 104 or 106by using different wavelengths of light within the same fiber opticcable.

Within the first host system 101A is one or more fiber optic transceivermodules 110A-110N. Similarly, in the second host system 101B are one ormore fiber optic transceiver modules 110A′-110N′. Each of the fiberoptic transceiver modules 110A-110N, 110A′-110N′ may include atransmitter T 120 and a receiver R 122 in order to providedbi-directional communication. If unidirectional communication isdesirable, a transmitter T 120 on one side and a receiver R 122 on theopposite side may be utilized instead of a transceiver having both.

Photons or light signals (e.g., data) are generated by the transmitter T120 in the first host system 101A; transmitted through the fiber opticcable of the link 104; and received by the receiver 122 of the secondhost system 101B. On the other hand, transmitter T 120 of the secondhost system 101B can generate photons or light signals (e.g., data) andtransmit them through the fiber optic cable of the link 106 which canthen be received by the receiver R 122 of the first host system 101A.Thus, the communication system 100 can utilize photons or light signalsto bi-directionally communicate data through the fiber optic cables andthe respective links between the first and second host systems101A,101B.

Referring now to FIG. 1B, a block diagram of the basic elements found ina fiber optic transceiver 110 are illustrated. Typically, a fiber optictransceiver 110 includes an electrical element (EE) 130, anelectro-optic element (EOE) 132, an optical element (OE) 134, and amechanical element (ME) 136 which interface with each other. Thetransmitter, a semiconductor diode or a semiconductor laser, and thereceiver, a photo-detector or photo-diode, are elements of the EOE 132and interface with the EE 130 and the OE 134. The OE 134 typicallyincludes one or more lenses or an optical block that includes lenses andpossibly reflective or refractive surfaces, or other passive opticalelements. The OE 134 couples light or photons between the fiber opticcable and the EOE 132. For example, a lens is typically used to couplelight into a fiber optic cable from a semiconductor laser and a lens istypically used to decouple light from a fiber optic cable into aphotodetector. The ME 136 typically includes the mechanisms used toalign the fiber optic cable with the one or more lenses and thetransmitter/receiver, the host electrical connector/connection (e.g., anedge connection of a printed circuit board) for the EE 130, as well asthe physical packaging and any mounting or release mechanism utilized incoupling the module to the host system. In that respect, the ME 136typically interfaces with all the elements of the typical fiber optictransceiver 110. In some cases, the elements of the fiber optictransceiver 110 may be split between elements for the transmitter 120and elements for the receiver 122. In other cases, the elements may beblended or joined, in order to provide support for both. For example,one or more components the electrical element may provide support forboth the transmitter 120 and elements for the receiver 122.

Referring now to FIG. 1C, a second exemplary fiber optic communicationsystem 100′ is shown. The fiber optic communication system 100′ is along haul fiber optic communications channel with one or more repeaters111A-111N between the ends of the communications channel. From a firsttransmitter 120′ to the first repeater 11A is a first fiber optic cable104′. Between repeaters 111A-111N are fiber optic cables 114A-114M.Between the last repeater 111N and the last receiver 122′ is anotherfiber optic cable 104″. The lengths of the fiber optic cable 104′, fiberoptic cables 114A-114M, and fiber optic cable 104″ are typically aslarge as possible in order to reduce the number of repeaters.

Each repeater 111A-111N includes a receiver 122 electrically coupled toa transmitter 120. In one embodiment, each repeater 111A-111N may be atransceiver 110 with received data from the receiver 122 coupled to thetransmitter 120 for retransmission.

FIG. 1C illustrates a uni-directional channel from transmitter 120′ toreceiver 122′. However, the fiber optic communication system 100′ can bereadily expanded to support bi-directional communication be duplicatingthe components and flipping them into reverse order from the end withthe receiver 122′ to the end with the transmitter 120′.

FIG. 2 illustrates a high level block diagram of the electrical elementswithin fiber optic transceiver modules of an fiber optic communicationsystem 200, an embodiment of the present invention. The fiber opticcommunication system 200 has an optical communication channel 202between a first fiber optic transceiver module 210 and a second fiberoptic transceiver module 210′.

The second fiber optic transceiver module 210′ is similar to the firstfiber optic transceiver module 210 but couple differently to the fiberoptic cables 204, 206. In the transmit data path, each fiber optictransceiver module 210,210′ includes a forward error correction (FEC)encoder 220, a pulse-shaping transmitter 222, and an electrical-optical(EO) converter 224, such as a semiconductor laser or otheropto-electronic transmitter. The pulse-shaping transmitter 222 mayinclude a spread-pulse modulator and be referred to as a spread-pulsemodulation transmitter (SPM TX). In the receive data path, each fiberoptic transceiver module 210,210′ includes an optical-electrical (OE)converter 232, a spread-pulse (SP) matched filter (MF) 234, an equalizer236, and a forward error correction (FEC) decoder 240. While datasamples b₀ are the transmitted data samples input into the FEC encoder220, data samples b₀″ out of the FEC decoder 240 are the received datasamples that are recovered from the optical communication channel.

FIG. 3A illustrates a functional block diagram of the electricalelements in a fiber optic data link between fiber optic transceivermodules of an fiber optic communication system 300. The fiber opticcommunication system 300 includes a transmitter 301, an optical channel302, and a receiver 303. The transmitter 301 includes a run-lengthlimited (RLL) encoder 310, a partial response (PR) precoder 312, a pulsefilter 314, and electrical-optical (EO) converter 316 coupled togetheras shown. The receiver 303 includes an optical-electrical (OE) converter320, an automatic gain control (AGC) 322, a spread-pulse (SP) matchedfilter 324, a timing recover phase locked loop (PLL) 326, a partialresponse (PR) finite impulse response (FIR) equalizer 328, a maximumlikelihood sequence estimation (MLSE) detector 330, a partial response(PR) postcoder 331, a summer 332, and a run-length limited (RLL) decoder334 coupled together as shown.

Referring now to FIGS. 3A and 3B, operation of the transmitter 301 isdescribed upon the start of data transmission at block 350. At thetransmitter 301, transmit data Dtx is coupled into a run-length limited(RLL) encoder 310 at a code rate of R=m/n (m bits of data are mappedinto n-bit codeword, e.g., 64/66) chosen to fit the given constraints ofthe optical channel 302. The RLL encoder encodes the transmit data Dtxinto RLL encoded data at block 352. The RLL encoding of the transmitdata Dtx facilitates synchronization at the receiver (i.e., its selfclocking), limits the effects of intersymbol interference (ISI) causedby channel dispersion in the optical channel 302, and reducespattern-dependent penalty. The RLL code may be described by parameters dand k, the respective minimum and maximum number of zeroes between ones(e.g., modified frequency modulation (MFM) code with d=1 and k=3).

Next, the RLL encoded data output from the RLL encoder 310 is coupledinto the PR precoder 312. The RLL encoded data is precoded into precodedata to prevent error propagation in the receiver 303 at block 354. Theprecoder 312 is designed to prevent catastrophic error propagation atthe receiver. The precoder 312 recursively correlates a sequence of bitsof the stream of RLL encoded data so that there is a dependency betweenthe data bits of the precoded data at the transmitter. That is, asequence of data bits in the precoded data stream are correlated to eachother. When received at the receiver, the preceding deters errorspropagation during decoding. In one embodiment of the invention, theprecoder may implement the equation y(n)=x(n)⊕y(n−2) for example wherey(n) is the output of the precoder for sample number n, x(n) is the datainput to the precoder for sample number n, y(n−2) is the output of theprecoder for sample number (n−2), and the symbol ⊕ represents anexclusive-or logical function. In another embodiment of the invention,the precoder may implement the equation y(n)=x(n)⊕y(n−1)⊕y(n−2), forexample. It is readily obvious that other equations may be implementedto correlate bit sequences together at the precoder 312, including usingmore orders as well as higher orders of correlation to correlate morebits and use an exclusive-nor logical function to perform the digitalbit correlation in place of the exclusive-or logical function.

Next, the precoded signal output from the precoder is coupled into thespread-pulse modulator 314. The spread-pulse modulator 314 is designedto fit a suitable pulse response (e.g., Gaussian or raised cosine). Thespread-pulse modulator 314 shapes the pulses of the precoded signal tospread out the pulses into a spread-pulse signal output at block 356 andmay be considered to perform spread pulse coding (SPC) or spread-pulsemodulation (SPM). The pulses may be spread beyond the bit intervalsprior to transmission in order that the eye is closed at thetransmitter. By spreading out the pulses in the spread-pulse signal,less distortion may be added by the optical channel 302 (i.e., thechannel response H(w)) during transmission. The pulse shape remainsnearly unchanged during the transmission over the optical channel. Byspreading out the pulses in the time-domain, (reducing the spread ofpulses in the frequency domain), the bandwidth of the original signal isreduced, the dispersion length (L_(D)=T₀ ²/B₂) is increasedsignificantly, and the dispersion effects of the optical fiber are thussubstantially eliminated. Additionally, spread pulse coding (i.e., pulsespreading or spreading out pulses) is immune to non-linear distortionscaused by the Kerr effect such as self-phase and cross-phase modulationand in PM-AM conversion. This immunity to nonlinear effects allows forhigher launch power, and therefore higher SNR at the receiver, withoutany significant loss in performance. Additionally the pulse spreadingallows for an exact matched filter design in the receiver that improvessignal to noise ratios. Finally, due to its bandwidth-narrowingproperty, SPC (or SPM) allows for tighter WDM channel spacing. CurrentWDM system employ a 100 GHz channel separation, with this design a 25GHz or less channel spacing is possible.

In one embodiment of the invention, the spread pulse modulator 314 isimplemented as a pulse-shaping filter 314 such as an analog Besselfilter. In another embodiment of the invention, the pulse-shaping filter314 is an analog raised cosine filter. The parameters of the filters(e.g., order, bandwidth) are selected to minimize the bit-error rate atthe receiver. In implementation, the pulse-shaping filter 314 may beimplemented in the optical domain by using a dispersive elementpositioned after the electrical to optical element 316 in one embodimentof the invention. In another embodiment of the invention, thepulse-shaping filter 314 is implemented in both the electrical domainand the optical domain. In another embodiment of the invention, thefunction of the pulse-shaping filter 314 is integrated within the EOElement 316. In yet another embodiment of the invention, thepulse-shaping filter 314 may be unused and omitted.

The signal output from the spread-pulse modulator 314, an electricalsignal, is coupled into the electrical-to-optical (EO) converter 316.The electrical-to-optical (EO) converter 316 is typically asemiconductor laser with a semiconductor laser driver (directmodulation) or external modulator. The spread pulse signal is used tomodulate the laser output of the semiconductor laser (i.e., theelectrical-to-optical (EO) converter 316) in order to transmit data overthe optical channel. Basically, the EO converter 316 converts thespread-pulse signal from an electrical signal in the electrical domaininto an optical or light signal in the optical domain as indicated byblock 358.

At block 359, the optical signal from the EO converter 316 is coupledinto an optical fiber of the optical channel 302 to transmit thespread-pulse signal over the optical fiber from the transmitter 301 tothe receiver 303. The optical or light signal of the transmittedspread-pulse signal experiences the channel response H(w) over theoptical channel 302.

Referring now to FIGS. 3A and 3C, operation of the receiver 303 isdescribed upon the start of data reception at block 360. At the receiver303, light or optical signals in the optical domain are received fromthe optical fiber at block 362. These received light or optical signalsrepresent a received spread-pulse signal. The light or optical signalsare coupled into the optical-to-electrical (OE) converter 320.

Then, the optical-to-electrical (OE) converter 320 converts the lightsignals into electrical signals representing the received spread-pulsesignal at block 364. The received spread-pulse signal, an electricalsignal in the electrical domain, is coupled into the AGC 322.

The AGC 322 provides gain for low amplitude signals and attenuation forhigh amplitude signals to limit or maintain the signal within a knownrange of amplitudes and keep the power level in the signal somewhatconstant as indicated by block 365. The automatic gain control enhanceslinearity in the system by reducing distortion and preventingsaturation.

The gain-controlled signal output from the AGC 322 is coupled into thematched filter 324. The matched filter 324 may be implemented either asa digital filter or an analog filter. The matched filter 324 is designedto have a response that closely matches the combined transmitter/channelresponse H(w) so as to optimize the signal to noise ratio in thepresence of noise. The matched filter 324 increases the signal-to-noiseratio of the receiver by filtering the received spread-pulse signalusing a matched filter as indicated by block 366.

A matched filter typically has a response which maximizes the signal tonoise ratio in the presence of white noise. To optimize the performanceof the receiver 303, knowledge of the channel transfer function is key.The optical channel is treated as being weakly non-linear. The lineareffects of the optical channel, such as dispersion and loss, dominate inthe early part of a pulses journey down the optical channel. The channelnon-linearities are included after the pulse disperses. The matchedfilter 324 is designed to fit a newly found transfer function thataccurately describes the envelope of the fiber optic channel. In oneembodiment of the invention, the matched filter 324 is an analog filterthat is matched to the spread pulse filter 314. In which case, thetransfer function used to describe the envelope of the fiber opticchannel is a time domain linear solution given by equation of A(z,t)below where the square of the pulse width is much less than B₂z.${A( {z,t} )} \approx {\frac{\overset{\sim}{A}( {0,\frac{t}{B_{2}z}} )}{\sqrt{2\pi\quad B_{2}z}}{\exp( {{- i}\frac{t^{2}}{2B_{2}z}} )}}$where T₀ ² is much less than B₂z.

A(z,t) is the pulse response at a distance z away from the transmitterwithin the channel (e.g., the fiber) at a time t. Ã(0,t/B₂z) is theFourier transform of A(0,t), the initial pulse at the transmitter (i.e.,z=0) evaluated at the frequency f equal to t/B₂z. The matched filter 324solves the dispersion problem in the channel (e.g., the fiber) ignoringnon-linear problems. Using this response equation, the matched filter324 can be simple, requiring no integration. The matched filter 324 isprogrammable based on channel properties such as distance z, dispersionfactor of channel (e.g., the fiber) B₂, and initial pulse width T₀.

The output of the matched filter 324 is also coupled into the input ofthe timing recovery PLL 326. From the signal output of the matchedfilter 324, the timing recovery PLL 326 generates or recovers a clocksignal as indicated by block 367 to synchronize data recovery functionstogether. The clock signal is coupled to the partial response (PR)finite impulse response (FIR) equalizing filter 328, the maximumlikelihood sequence estimation (MLSE) detector 330, such as a Viterbidetector, the AGC 322, the PR postcoder 331, and the RLL decoder 334. Inthis manner the timing of the partial response (PR) finite impulseresponse (FIR) equalizing filter 328, the maximum likelihood sequenceestimation (MLSE) detector 330, the AGC 322, the partial responsepostcoder 331, and the RLL decoder 334 may be synchronized together.

The output of the matched filter 324 is coupled into the input of thepartial response equalizing (PR) filter 328. The PR filter 328 is anadaptive filter that can be implemented as either an analog filter, adigital filter, or a combination thereof. The partial response filter328 shapes the spectrum of the incoming signal from the channel, thereceived spread-pulse signal, into that of a desired partial-responsesignal at block 368. That is the partial response filter 328 shapes thereceived spread-pulse signal into a desired target response, thepartial-response signal, in order to reduce distortion by equalizing thelinear distortion that may have been introduced by the channel. In oneembodiment, the partial response filter 328 is an adaptive finiteimpulse response (FIR) filter that can adapt to track variations in thechannel response. The partial response filter 328 allows a controlledamount of intersymbol interference to be left in the equalizedpartial-response signal. This avoids zero-forcing equalization found ininverse channel equalization. The partial response filter 328 also doesnot suffer from noise enhancement and instability typically encounteredin inverse channel equalization. Since, the partial response filter 328is implemented as a FIR filter, it may be referred to as a linearequalizer.

Referring now to FIG. 4, an adaptive finite impulse response (FIR)filter 400 is illustrated as one embodiment of the partial responsefilter 328. The FIR filter 400 includes N delay elements 402A-402N, N+1FIR filter coefficients 404A-4040, and an adder or summer 406 coupledtogether as illustrated. The N delay elements 402A-402N may beimplemented as a register delay in the data path. The N+1 FIR filtercoefficients 404A-4040 are multiplied together with the respectivedelayed data input to generate the terms of the equation using a boothmultiplier or a recursive adder, for example. The adder 406 sums theterms of the equation together to generate the output response y(n).

The adaptive FIR filter 400 implements the following equation:${y(n)} = {\sum\limits_{k = 0}^{L}{W_{k}{x( {n - k} )}}}$

The W_(k) represents the N+1 FIR coefficients 404A-4040, the value of Lis the FIR filter order less one, x(n−k) is the input, and y(n) is theoutput.

The partial-response signal (e.g., (1+D) partial-response signal) may bedescribed by the following equation:${Y(D)} = {\sum\limits_{k = 0}^{k = {l - 1}}{x^{k}D^{k}}}$

The order (l) and coefficients (x_(k)) in the equation of thepartial-response signal are chosen to fit the constraints of a givenfiber optic channel. The order (l) and coefficients (x_(k)) aretypically whole numbers. If the optical channel is expected to generatesevere inter-symbol interference, real-valued coefficients (x_(k)) maybe used.

In one embodiment, the order is two (i.e., l=2, and Y(D)=x₀+x₁D) and thecoefficients are set to one (i.e., x₁=x₀=1) such that the equation Y(D)simplifies to (1+D) and is the duobinary partial response signal. Inanother embodiment, the order is three (i.e., l=3, Y(D)=x₀+x₁D+x₂D²) andthe coefficients are set as x₁=2, x₂=x₀=1) such that the equation Y(D)simplifies to (1+2D+D²) and is the type 2 partial response signal.

Next, the equalized partial response signal (i.e., the output of thepartial-response filter) is coupled to the input of the maximumlikelihood sequence estimation (MLSE) detector 330 and a first input ofthe summer 332. In one embodiment, the MLSE detector is a Viterbidetector. As discussed previously, the PR FIR filter 328 allows someintersymbol interference (ISI) in the equalized partial response signal.That is, adjacent data transitions in the equalized partial responsesignal may interfere with each other. At block 370, the MLSE detector330 removes the remaining intersymbol interference (ISI) from theequalized partial response signal to generate an MLSE data signal,corresponding to correlated RLL coded data. As the MLSE detector 330performs a nonlinear function, it may also be referred to as anon-linear equalizer. A multi-stage process of equalization is providedby embodiments of the invention in that the PR FIR filter provideslinear equalization and the MLSE detector 330 provides non-linearequalization.

Referring now to FIGS. 5A-5B, the operation the MLSE detector 330 with atwo state data input of zero and one is now described. As discussedpreviously, the MLSE detector 330 is implemented as a Viterbi detectorin one embodiment of the invention.

Assume that in the partial response equation Y(D) the order is two(i.e., l=3) and the coefficients are set as (x₂=x₀=1, x₁=1). In thiscase, the ideal partial response equation Y(D) simplifies to (1+D)² or1+2D+D². FIG. 5A illustrates a time domain functional block diagram 500to implement the ideal partial response equation of Y(D)=1+2D+D². Thefunctional block diagram 500 includes time delay elements 502A-502B, adoubler (×2) 504, and adders 506A-506B coupled together as shown in FIG.5A. Assuming digital components are used, the delay elements 502A-502Bmay simply be implemented as clocked D type flip flops. The doubler (×2)504 may be implemented as a digital multiplier or a binary bit shifter.The adders 506A-506B may simply be implemented as a pair of two bitdigital adders.

In implementing the ideal partial response equation of Y(D)=1+2D+D², theinput sample x(n) has data bits of 0 and 1 and can generate five levelsof output (0, 1, 2, 3, and 4) as the output y(n). The PR filter 328 ofFIG. 3A is designed to produce a signal that is as close as possible tothe ideal partial response signal Y(D). The PR filter 328 produces aversion of the signal Y(D) that is corrupted with some noise andimperfections of the filter implementation. The MLSE detector 330samples the output of the PR FIR filter 328 (i.e., the noisy version ofthe ideal partial response signal Y(D)) in order to recover the inputdata signal x(n) on each clock transition.

FIG. 5B illustrates a trellis diagram with all four possible outputstates for the partial response equation of Y(D)=1+2D+D² and its fivelevels of generated output with the input sample x(n) having data bitsof 0 and 1. The four possible output states are State OO, State 01,State 10, and Stage 11. The MLSE detector 330, accumulates over Niterations (known as the memory of the MLSE) a distance metric (ameasure comparing the received signal with the ideal signal) over eachpossible path in the trellis and selects the path that has the smallestaccumulated distance. The input data signal x(n) is recovered by tracingback the optimal path (the one with the shortest distance) and itscorresponding input symbols. For example, consider at time t0 that thecurrent state is ‘11’, an input symbol ‘0’ at time t0 would produce theoutput symbol ‘3’ and the new state ‘01’. If at time t0, the inputsymbol is ‘1’ then the next output state and output symbol would be ‘11’and ‘4’, respectively.

The MLSE detector 330, knowing the current output state at time t0 andin response to the input data x(n) and the output level y(n) at time t0,transitions to a next output state at time t1. The input data x(n) andthe output level y(n) at time t0 are respectively represented in an I/Oformat along each line. For each current output state at time t0, thereare two I/O combinations that may cause the MLSE detector to the nextoutput state at time t1.

For example, consider at time t0 that the current output state is astate 01. I/O combinations of 0/1 or 1/2 for x(n)/y(n) respectivelycause a state 00 or state 10 to be generated as the next output state attime t1. Now consider at time t0 that the current output state is astate 00, for example. I/O combinations of 0/0 or 1/1 for x(n)/y(n)respectively cause a state 00 or state 10 to be generated as the nextoutput state at time t1. In this manner, the current output state aswell as a number of weighted input samples can effect the next outputstate of the MLSE detector such that intersymbol interference may beeliminated from the output.

Referring now to FIGS. 6A-6C, the operation of the MLSE detector 330with a general data input of positive a (+a) and negative a (−a) is nowdescribed. As discussed previously, the PR filter 328 of FIG. 3A isdesigned to produce a signal that is as close as possible to the idealpartial response signal Y(D)=1+2D+D². The MLSE detector 330, accumulatesover N iterations (known as the memory of the MLSE) a distance metric (ameasure comparing the received signal with the ideal signal) over eachpossible path in the trellis and selects the path that has the smallestaccumulated distance. The input data signal x(n) is recovered by tracingback the optimal path (the one with the shortest distance) and itscorresponding input symbols. FIG. 5A illustrates a data signal input of0 and 1 for x(n) that is substituted for by a general data input ofpositive a (+a) and negative a (−a) for x(k). In implementing the idealpartial response equation in this case, an input sample x(k) has ageneral data input of positive a (+a) and negative a (−a) that cangenerate five levels of output (0, 2a, 4a, −2a, and −4a) as the outputy(k) substituted for y(n) in FIG. 5A. The FIR filter is designed toproduce a signal that closely resembles the ideal response y(n). Theoutput of the FIR filter, that is y(k) corrupted with some noise, iscoupled into the MLSE detector 330.

The PR filter 328 produces a version of the signal Y(D) that iscorrupted with some noise due to the imperfections of the filterimplementation. The MLSE detector 330 samples the output of the PR FIRfilter 328 (i.e., the noisy version of the ideal partial response signalY(D)) in order to recover the input data signal x(k) on each clocktransition. The MLSE detector 330 is implemented as a Viterbi detectorin one embodiment of the invention.

In FIG. 6A, a multi state trellis state diagram is illustrated for thesecond order partial response signal encoding of FIG. 5A with generaldata input symbols of positive a (+a) and negative a (−a). The currentoutput state (state 0, 1, 2, 3) of the MLSE detector 330 is to the leftof the trellis state diagram at time t₀=(k−1). To the right of thetrellis state diagram is the next output state (state 0, 1, 2, 3) attime t₁=(k) to which the output of the MLSE Detector may change inresponse to the current input x(k) and the output y(k). Just to theright of each state is a metric notation m_(j)(k−1) or m_(j)(k). Thenotation m_(j)(k) represents the value of the metric at state j and timek. The metric notation m_(j)(k−1) or mj(k) represent equations that areused to determine the transition to the next output state from a currentstate. That is, given a current state j at time t=k−1 and the value ofmetric m_(j)(k−1), two new metrics, corresponding to two possibletransitions from state j, are computed using the newly received sampley(k). This process is repeated for each state.

In FIG. 6B, equations are illustrated of the metric update algorithm forthe multi state trellis state diagram of FIG. 6A. Four equations of themetric update algorithm are provided in FIG. 6A including m₀(k), m₁(k),m₂(k), m₃(k) corresponding to the metrics of states 0,1,2,3 at time k inorder to determine a value for each. In each equation, y(k) denotes thereceived signal at time k, Min{} denotes taking the minimum of the twovalues in the set to be the value for the metric, and “a” is the valueof the general data input. Each of these equations is evaluated at timek using the past value at time t₀=(k−1) in order to determine the nextoutput state as well as to be updated for a determination of the outputstate that follows after. In the equations, various threshold values areused to and added to the prior state in order to determine the currentstate. For example a threshold value of y(k)+a is added to m₁(k−1) inthe second term in the set for the equation of m₀(k). As anotherexample, a threshold value of −2y(k)+4a is added to m₃(k−1) in thesecond term in the set for the equation of m₃(k). Instead of determininga minimum value between two terms in the set, an advanced determinationmay be made as to which of the two values within a set will be theminimum value in order to simplify and reduce the computations of eachmetric. In this manner, only one of the two terms need to be computed inorder to update the respective metric.

FIG. 6C illustrate a chart of the conditions used to implement theequations of the metric update algorithm illustrated in FIG. 6B for themulti state trellis state diagram of FIG. 6A. The chart illustrated inFIG. 6C makes an advanced determination as to which of the two valueswithin a set will be the minimum value in order to simplify and reducethe computations of each metric. Three columns are illustrated in FIG.6C. In the left column, conditions are provided in which a comparison ismade with Δm₀₁(k−1) and Δm₂₃(k−1) against threshold values . Thenotation m₀₁(k−1) refers to evaluating the equation ofΔm₀₁(k−1)=m₀(k−1)−m₁(k−1) and the notation Δm₂₃(k−1) refers toevaluation the equation of Δm₂₃(k−1) =m₂(k−1)−m₃(k−1). In the centercolumn, equations to update the metrics m₀(k), m₁(k), m₂(k), m₃(k) attime k are provided in response to the conditions indicated in the leftcolumn. In the right column of the chart, paths to select from thecurrent state (shown on the left) to the next state (shown on the right)are provided in response to current state and the metric values of thecenter column given the conditions of the left column.

In updating the metric m₀(k), a determination is made whether or notΔm₀₁(k−1) is less than the threshold value of −y(k)−3a. If so, then themetric m₀(k) is updated using the equation m₀(k)=m₀(k−1)+2y(k)+4a fromthe center column. If not, then the metric m₀(k) is updated using theequation m₀(k)=m₁(k−1)+y(k)+a.

In updating the metric m₁(k), a determination is made whether or notΔm₂₃(k−1) is less than the threshold value of −y(k)+a. If so, then themetric m₁(k) is updated using the equation m₁(k)=m₂(k−1) from the centercolumn. If not, then the metric m₁(k) is updated using the equationm₁(k)=m₃(k−1)−y(k)+a.

In updating the metric m₂(k), a determination is made whether or notΔm₀₁(k−1) is less than the threshold value of −y(k)−a. If so, then themetric m₂(k) is updated using the equation m₂(k)=m₀(k−1)+y(k)+a from thecenter column. If not, then the metric m2(k) is updated using theequation m₂(k)=m₁(k−1).

In updating the metric m₃(k), a determination is made whether or notΔm₂₃(k−1) is less than the threshold value of −y(k)+3a. If so, then themetric m₃(k) is updated using the equation m₃(k)=m₂(k−1)−y(k)+a from thecenter column. If not, then the metric m₃(k) is updated using theequation m₃(k)=m₃(k−1)−2y(k)+4a.

In selecting a path given a current state of 0, the next state is 0 ifΔm₀₁(k−1) is less than the threshold value of −y(k)−3a. Otherwise, theother path for the current state of 0 is selected to go to a next stateof 2.

In selecting a path given a current state of 1, the next state is 0 ifΔm₀l(k−1) is less than the threshold value of −y(k)−3a. Otherwise, theother path for the current state of 1 is selected to go to a next stateof 2.

In selecting a path given a current state of 2, the next state is 1 ifΔm₂₃(k−1) is less than the threshold value of −y(k)+a. Otherwise, theother path for the current state of 2 is selected to go to a next stateof 3.

In selecting a path given a current state of 3, the next state is 1 ifΔm₂₃(k−1) is less than the threshold value of −y(k)+a. Otherwise, theother path for the current state of 3 is selected to go to a next stateof 3.

In this manner, the output of the MLSE detector may be determined andthe metrics can be updated for future state determination by computingvalues of a few equations and performing a few comparisons againstthreshold values.

Referring back now to FIGS. 3A and 3C, the operation of the receiver 303is further described. The output of the MLSE detector 330 (i.e., theMLSE data signal corresponding to correlated RLL coded data) is coupledto the input of the PR postcoder 331 (and a second input of the summer332). The PR postcoder 331 performs the inverse function of the PRprecoder 312. As discussed previously, the precoder 312 recursivelycorrelates a sequence of bits of the stream of RLL encoded data to avoiderror propagation at the receiver. That is, a sequence of data bits inthe precoded data stream are correlated to each other beforetransmission. Thus in the receiver, the PR postcoder 331 recursivelyde-correlates a predetermined sequence of bits in the MLSE data signal(corresponding to correlated RLL coded data) as indicated by block 371.The number of predetermined sequence of bits being de-correlated in thereceiver may match the number of the predetermined sequence of bits thatwere correlated in the transmitter. This removes the dependency betweendata bits in the data stream.

The output of the MLSE detector 330 (i.e., the MLSE data signal) is alsocoupled to the second input of the summer 332. The output of the summer332 is coupled into a tracking loop circuit 333. The summer 332functions as a subtractor to compare the input and output of the MLSEdetector together. The difference between the values at the input andoutput of the MLSE detector are coupled into the input of the trackingloop circuit 333.

The summer 332 and the tracking loop circuit 333 are in a feedback pathfrom the MLSE detector 330 to the PR FIR filter 328. The output of thetracking loop circuit 333, an error signal en, is coupled into the PRFIR filter 328. The error signal en is coupled to the PR FIR equalizingfilter 328 to adjust the coefficients of the filter.

The tracking loop circuit 333 keeps a running tab of the error betweenthe input and output of the MLSE detector generated by the summer 332.The error is used to adjust the coefficients of the FIR. In this manner,the FIR is able to track slow channel variations (such as due totemperature changes)

As discussed previously, the PR postcoder 331 performs the inversefunction of the PR precoder 312 on the signal output from the MLSEdetector 330. The de-correlated data output from the PR postcoder 331 iscoupled into the input of the RLL decoder 334. The RLL decoder 334recovers the transmitted data D_(TX) from the de-correlated MLSE datasignal as received data D_(RCV) at block 372. The RLL decoder 334 usesthe same run length limited code to decode data as was used by the RLLencoder 310 to encode data.

The RLL decoder 334 generates the received data D_(RCV) at block 372from the de-correlated MLSE data signal output generated by the PRprecoder 312 which completes the discussion of the data reception atblock 375. While RLL encoding and decoding is described and illustratedby the RLL encoder and RLL decoder, data may be transmitted without RLLencoding and thus may not require RLL decoding.

As the communication system spreads out the pulses using spread pulsecoding in the data transmission and performs partial responseequalization and maximum likelihood sequence estimation during datareception, the communication system may be referred to as a spread pulsepartial response maximum likelihood (SPPRML) communication system.

According to one embodiment of the invention, the transmitter 301 andthe receiver 303 may be implemented in one or more application specificintegrated circuits (ASICs). In this manner, the transmitter 301 and thereceiver 303 may include the functions of current dispersioncompensation modules (fiber or otherwise), Polarization Mode Dispersioncompensators, and clock and data recovery (CDR) circuits into anintegrated circuit solution.

Referring now to FIG. 7A, a first functional block diagram of elementswithin a fiber optic transceiver module 700A is illustrated. At theheart of the fiber optic transceiver module 700A is an applicationspecific integrated circuit (ASIC) 750A mounted to a printed circuitboard 701A. The application specific integrated circuit (ASIC) 750Aimplements a number of the previously described functions of thetransmitter 301 and receiver 303 in circuitry on a monolithic siliconsubstrate. The fiber optic transceiver module 700A further includes amicroprocessor 751, a retimer 752, an electrical-to-optical (EO)converter 716, and an optical-to-electrical (OE) converter 720 mountedto the printed circuit board 701A and coupled together with the ASIC750A as shown and illustrated in FIG. 7A. The electrical-to-optical (EO)converter 716 includes a linear laser driver 754 and a directly orexternally modulated semiconductor laser 756 coupled together as shown.The optical-to-electrical (OE) converter 720 includes a photodetector,such as a PIN photodiode, and a transimpedance amplifier (TIA).

On the electrical side, the fiber optic transceiver module 700A receivestransmit data (Tdata) and a clock signal and outputs received data(Rdata). On the optical side, the fiber optic transceiver module 700Areceives receive light pulses (RLP) from a first fiber optic cable andoutputs transmit light pulses (TLP) to couple into a second fiber opticcable.

Basically, the ASIC 750A spreads the transmit data (Txdata) and drivesthe optical channel by generating time-spread transmit data (PTxdata),an electrical signal which is to be converted into an optical signal(i.e., transmit light pulses (TLP)0 for transmission over the opticalchannel. The ASIC 750A further recovers the clock (referred to as arecovered clock, Rclk) and data (Rdata) from the received data (Rxdata),an electrical signal converted from the receive light pulses (RLP), thatwas processed at far-end and may have been slightly distorted by theresponse of the optical channel. In which case, the ASIC 750A may bereferred to as a preemphasis dispersion-tolerant ASIC 750A.

In the transmit data path, the preemphasis dispersion-tolerant ASIC 750Aincludes a run length limited (RLL) encoder 710, a PR precoder 712, anda spread-pulse modulator 714 coupled together as shown. The RLL encoder712, PR precoder 712, and spread-pulse modulator 714 respectivelyfunction similar to the RLL encoder 310, PR precoder 312, andspread-pulse modulator 314 as previously described with reference toFIGS. 3A and 3B.

In the receive data path, the preemphasis dispersion-toleranttransceiver ASIC 750A includes an automatic gain control (AGC) 722, amatched filter 724, a timing recover phase locked loop (PLL) 726, apartial response (PR) finite impulse response (FIR) equalizer 728 (i.e.,a linear equalizer), a maximum likelihood sequence estimation (MLSE)detector 730 (i.e., an nonlinear equalizer), and a run-length limited(RLL) decoder 734 coupled together as shown in FIG. 7A. The automaticgain control (AGC) 722, matched filter 724, timing recover phase lockedloop (PLL) 726, partial response (PR) finite impulse response (FIR)equalizer 728 (i.e., analog equalizer), maximum likelihood sequenceestimation (MLSE) detector 730 (i.e., a nonlinear equalizer), PRpostcoder 731, and run-length limited (RLL) decoder 734 respectivelyfunction similar to the automatic gain control (AGC) 322, matched filter324, timing recover phase locked loop (PLL) 326, partial response (PR)finite impulse response (FIR) equalizer 328, maximum likelihood sequenceestimation (MLSE) detector 330, the PR postcoder 331, and the run-lengthlimited (RLL) decoder 334 as previously described with reference toFIGS. 3A and 3C.

The preemphasis dispersion-tolerant transceiver ASIC 750A furtherincludes a diagnostic host interface 741, a pseudo random binarysequence (PRBS) generator 744, and a built in self tester (BIST) 746coupled together as shown in FIG. 7A. The diagnostic host interface 741couples to the microprocessor 751 to provide diagnostic information(e.g., status) to the microprocessor as well as register access toprovide the initial setup (i.e., initialization) for the preemphasisdispersion-tolerant transceiver ASIC 750A. The diagnostic host interface741 may also be used to signal the microprocessor when an error isdetected by the preemphasis dispersion-tolerant transceiver ASIC 750A.

The pseudo random binary sequence (PRBS) generator 744 and the built inself tester (BIST) 746 are used to test the communication channel fromone fiber optic transceiver module to the next as well as to provide aself test of the preemphasis dispersion-tolerant transceiver ASIC 750Asuch as upon power up. The pseudo random binary sequence generated bythe pseudo random binary sequence (PRBS) generator 744 is coupled to theprecoder 712 and the BIST 746. The BIST 746 also is coupled to the RLLdecoder 734 to receive the looped back test data for the purpose ofcomparison with the pseudo random binary sequence generated by thepseudo random binary sequence (PRBS) generator 744. If the preemphasisdispersion-tolerant transceiver ASIC 750A is to be self tested, the datais looped back before being transmitted over the channel. If the overallcommunication channel is to be tested, the data may be looped back atthe opposite end of the communication channel.

Referring now to FIG. 7B, a second functional block diagram of elementswithin a fiber optic transceiver module 700B is illustrated. The fiberoptic transceiver module 700B is similar to the fiber optic transceivermodule 700A but for pulse shaping block 714. The spread-pulse modulatorblock 714 is moved out of the ASIC 750A, resulting in ASIC 750B, andinstead a spread-pulse modulator 714′ is mounted on the printed circuitboard 701B and coupled between a laser driver 754′ and the directly orexternally modulated semiconductor laser 756 in the case of directmodulation or between a laser driver and an external modulator in thecase of external modulation. The driver 754′, spread-pulse modulatorblock 714′, and the directly or externally modulator/semiconductor laser756 are coupled together as shown to form an electrical-to-optical (EO)converter 716′. Thus, the preemphasis dispersion-tolerant transceiverASIC 750B slightly differs from the preemphasis dispersion-toleranttransceiver ASIC 750A with the function of the similar blocks beingdescribed above with reference to FIG. 7A and not repeated here forreasons of brevity.

Referring now to FIG. 8, an exemplary fiber optic transceiver module 810is illustrated. The fiber optic transceiver module 810 includes anintegrated circuit 850 mounted therein to a printed circuit board 860that incorporates embodiments of the invention. As discussed previously,the integrated circuit 850 may be one or more application specificintegrated circuits (ASICs) to support both the electronics of thetransmitter 301 and the receiver 303. The fiber optic transceiver module810 further includes a light transmitter 820 (i.e., an EO converter) anda light receiver 822 (i.e., an OE converter). The fiber optictransceiver module 810 may be compatible with the 10 gigabit per second(10 GPS) small form-factor pluggable multi-source agreement (XFP), thethree hundred pin multi-source agreement (MSA), XPAK, X2, XENPAC andother proprietary or standard packages.

The printed circuit board 860 includes top and bottom pads (top pads 872illustrated) to form an edge connection 870 to couple to a socket of ahost printed circuit board. A housing 812 couples around the printedcircuit board 860 to protect and shield the integrated circuit 860. Afront fiber optic plug receptacle 840 is provided with openings 842 tointerface with one or more fiber optic cables and their plugs. Amechanical latch/release mechanism 830 may be provided as part of thefiber optic transceiver module 810. While the fiber optic transceivermodule 810 has been described has having both light transmission andlight reception capability, it may be a fiber optic transmitter modulewith light transmission only or a fiber optic receiver module with lightreception only.

Referring now to FIG. 9A, a waveform diagram of first simulation resultsis illustrated with the y-axis representing amplitude and the x-axisrepresenting time or the number of data samples for the givenpulse-width. In FIG. 9A, a transmit signal 901 with a pulse width of 250picoseconds and a clock period of 100 picoseconds is launched into a 500kilometer single mode fiber (SMF) using the embodiments of theinvention. The transmit signal 901 is measured at output of theelectrical-optical converter (EO) 316 illustrated in FIG. 3A. A receivedsignal 903 is measured at the input to the optical-electrical converter(OE) 320 illustrated in FIG. 3A. With a pseudo random binary sequence(PRBS) of 100 bits in the embodiments of the invention, the receivedsignal 903 tracks the transmit signal 901 very well such that dispersioneffects are substantially reduced. That is, the optical channel 302 addslittle distortion to the transmit signal 901 that is received as thereceive signal 903 at the receiver 303. This is because the transmitsignal 901 has been spread (preconditioned) as previously described inorder to avoid the distortion of the optical channel. With littledistortion from the channel, data can be readily recovered from thereceive signal 903.

Referring now to FIG. 9B, a waveform diagram of second simulationresults is illustrated with the y-axis representing amplitude and thex-axis representing time or the number of data samples for the givenpulse-width. In FIG. 9B, a transmit signal 910 with a pulse width of 250picoseconds and a clock period of 100 picoseconds is launched into a 600kilometer single mode fiber (SMF) using the embodiments of theinvention. The transmit signal 910 is measured at output of theelectrical-optical converter (EO) 316 illustrated in FIG. 3A. A receivedsignal 913 is measured at the input to the optical-electrical converter(OE) 320 illustrated in FIG. 3A. With a pseudo random binary sequence(PRBS) of 100 bits, the received signal 912 using the embodiments of theinvention tracks the transmit signal 910 very well such that dispersioneffects are substantially reduced. Again the optical channel 302 addslittle distortion to the transmit signal 910 that is received as thereceive signal 913 at the receiver 303. This second simulation of FIG.9B differs from the first simulation of FIG. 9A in that the opticalfiber distance has increased by 100 kilometers, from 500 to 600kilometers, with little added distortion. For comparison, typicallengths of fiber optic cables between repeaters without the embodimentsof the invention are on the order of 40 to 80 kilometers for externallymodulated lasers and less than 10 Km for direct laser modulation.

The embodiments of the invention conserve energy in opticalcommunication systems. The embodiments of the invention employ mixedsignal circuitry, a combination of analog and digital circuits, insteadof pure digital circuitry. This reduces the number of active circuitsover that of a pure digital circuit implementation that would require alarge number of active digital logic gates. The embodiments of theinvention further eliminate the need for dispersion compensating fiber(DCF) and its associated active circuitry (i.e., optical amplifiers) tofurther lower the overall power consumption of the transmission system.Moreover, as the length of transmission may be increased by using theembodiments of the invention, fewer repeaters may be needed to transmitdata over a given path. In light of the significant number of fiberoptic communication systems deployed in the United States and thefurther increasing use of fiber optic communication systems, theembodiments of the present invention may materially reduce the amount ofpower consumed, the required footprint and may have an impact upon theoverall electrical energy consumption used by all the fiber opticnetworks which are in use today.

The embodiments of the invention may be applied to a number of opticaldigital communications systems, including but not limited to SONET, SDH,Ethernet, metro, long haul, ultra-long haul, and submarine opticalcommunications systems. The embodiments of the invention are applicableto all bit or data rates used in a communication system (e.g., 1 Gbps,2.5 Gbps, 10 Gbps, and 40 Gbps) and to all types of optical fibers(e.g., Non Dispersion Shifter Fiber (NDSF), Non-Zero Dispersion ShiftedFiber (NZ-DSF, a.k.a. Lambda-Shifted Fiber), Dispersion Shifter Fiber(DSF), single mode optical fiber (SMF), and multi-mode optical fiber(MMF)). Additionally, the laser transmitter may be a cooled ornon-cooled laser. Embodiments of the invention may directly modulate adirect modulated laser (DML) or indirectly modulate an externalmodulated laser (EML) by driving an external modulator.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described. For example, embodiments of theinvention have been shown and described for use over an opticalcommunication channel in optical communication systems. However, theembodiments of the invention may be used in other dispersivecommunication channels or non-optical communication channels in othercommunication systems. That is, the embodiments, the embodiments of theinvention may be applied to metal wire communication systems thattransmit and receive electrical signals over a metal (e.g., copper wire)without electrical-to-optical (EO) conversion and optical-to-electrical(OE) conversion.

Additionally, it will be evident that various modifications and changesmay be made thereto without departing from the broader spirit and scopeof the present invention as set forth in the appended claims. Therefore,the specification and drawings are accordingly to be regarded in anillustrative rather than in a restrictive sense.

1-33. (canceled)
 34. An apparatus for an optical communication channel,the apparatus comprising: a run length limited data encoder having aninput to receive transmit data, the data encoder further having anoutput, the data encoder to encode the transmit data into coded data atthe output of the data encoder; a partial response precoder having aninput coupled to the output of the data encoder to receive the codeddata, the precoder further having an output, the precoder to correlatebits of the coded data together into a precoded signal at the output ofthe precoder; a pulse filter having an input coupled to the output ofthe precoder to receive the precoded signal, the pulse filter furtherhaving an output, the pulse filter to spread out the pulses in theprecoded signal into a spread-pulse signal at the output of thepulse-shaping filter; and an electrical-to-optical converter having aninput coupled to the output of the pulse filter, theelectrical-to-optical converter further having an optical output totransmit a first plurality of light pulses over the opticalcommunication channel, the electrical-to-optical converter to convertthe spread-pulse signal from an electrical signal into the firstplurality of light pulses at the optical output.
 35. The apparatus ofclaim 34, wherein the apparatus is a transmitter.
 36. The apparatus ofclaim 34, wherein the run length limited data encoder encodes thetransmit data into the coded data using a run length limited code. 37.The apparatus of claim 34, wherein the pulse filter spreads out thepulses of the precoded signal into the spread-pulse signal in accordancewith a raised cosine filter response.
 38. The apparatus of claim 34,wherein the pulse filter spreads out the pulses of the precoded signalinto the spread-pulse signal in accordance with a Gaussian filterresponse.
 39. The apparatus of claim 34, wherein theelectrical-to-optical converter includes, a laser driver having an inputcoupled to the output of the pulse filter, the laser driver to convertthe spread-pulse signal into a laser driver signal, and a semiconductorlaser having an input coupled to the output of the laser driver toreceive the laser driver signal, the semiconductor laser further havingthe optical output, the semiconductor laser to generate the firstplurality of light pulses at the optical output in response to the laserdriver signal.
 40. The apparatus of claim 34, further comprising: anoptical-to-electrical converter having an optical input to receive asecond plurality of light pulses from the optical communication channel,the optical-to-electrical converter further having an output, theoptical-to-electrical converter to convert the second plurality of lightpulses from an optical signal into an electrical signal as a receivedspread-pulse signal at the output of the optical-to-electricalconverter; an automatic gain control amplifier having an input coupledto the output of the optical-to-electrical converter, the automatic gaincontrol amplifier to limit an amplitude of the received spread-pulsesignal to a predetermined range; a spread pulse matched filter coupledto the automatic gain control amplifier to receive the receivedspread-pulse signal, the matched filter further having an output, thematched filter to filter the received spread-pulse signal to optimize asignal to noise ratio; a partial response (PR) finite impulse response(FIR) equalizing filter having an input coupled to the output of thematched filter, the PR FIR equalizing filter to equalize the receivedspread-pulse signal into a predetermined partial response signal; amaximum likelihood sequence estimation (MLSE) detector having an inputcoupled to an output of the PR FIR equalizing filter, the MLSE detectorto equalize the predetermined partial response signal and recover anencoded data signal; a partial response postcoder coupled to an outputof the MLSE detector, the partial response postcoder to recursivelyde-correlate a sequence of data bits in the encoded data signal; and arun length limited (RLL) data decoder having an input coupled to anoutput of the MLSE detector to receive the encoded data signal, the RLLdata decoder further having an output, the RLL data decoder to decodethe encoded data signal into received data at the output of the datadecoder.
 41. The apparatus of claim 40, wherein the apparatus is atransmitter/receiver or transceiver.
 42. The apparatus of claim 34,further comprising: an optical-to-electrical converter having an opticalinput to receive a second plurality of light pulses from the opticalcommunication channel, the optical-to-electrical converter furtherhaving an output, the optical-to-electrical converter to convert thesecond plurality of light pulses from an optical signal into anelectrical signal as a received spread-pulse signal at the output of theoptical-to-electrical converter; a matched filter coupled to theoptical-to-electrical converter to receive the received spread-pulsesignal, the matched filter further having an output, the matched filterto filter the received spread-pulse signal to optimize a signal to noiseratio; a partial response (PR) finite impulse response (FIR) equalizingfilter having an input coupled to the output of the matched filter, thePR FIR equalizing filter to equalize the received spread-pulse signalinto a predetermined partial response signal; a maximum likelihoodsequence estimation (MLSE) detector having an input coupled to an outputof the PR FIR equalizing filter, the MLSE detector to equalize thepredetermined partial response signal and recover an encoded datasignal; and a data decoder having an input coupled to an output of theMLSE detector to receive the encoded data signal, the data decoderfurther having an output, the data decoder to decode the encoded datasignal into received data at the output of the data decoder.
 43. Theapparatus of claim 42, wherein the apparatus is a transmitter/receiveror transceiver.
 44. The apparatus of claim 42, wherein theoptical-to-electrical converter includes, a photo-detector having anoptical input to receive the second plurality of light pulses from theoptical fiber, the photo-detector further having an output, thephoto-detector to convert the second plurality of light pulses from anoptical signal into an electrical signal at the output of thephoto-detector, and a transimpedance amplifier having an input coupledto the output of the photo-detector to receive the electrical signalfrom the photo-detector, the transimpedance amplifier further having anoutput, the transimpedance amplifier to amplify the electrical signalreceived from the photo-detector into the received spread-pulse signalon the output of the transimpedance amplifier.
 45. The apparatus ofclaim 42, wherein the matched filter has a response which optimizes thesignal to noise ratio of the received spread-pulse signal in thepresence of white noise.
 46. The apparatus of claim 42, wherein thematched filter is a filter having a transfer function with a pulseresponse substantially in accordance with${A( {z,t} )} \approx {\frac{\overset{\sim}{A}( {0,\frac{t}{B_{2}z}} )}{\sqrt{2\pi\quad B_{2}z}}{\exp( {{- i}\frac{t^{2}}{2B_{2}z}} )}}$where T₀ ² is much less than B₂z.
 47. The apparatus of claim 42, whereinthe matched filter is an analog filter that has a response substantiallymatched to a response of the pulse filter.
 48. The apparatus of claim47, wherein the pulse filter and the matched filter are Bessel filters.49. The apparatus of claim 42, wherein the MLSE detector is a Viterbidetector to equalize the predetermined partial response signal andrecover the encoded data signal.
 50. The apparatus of claim 49, whereinthe MLSE detector further to remove intersymbol interference (ISI) fromthe predetermined partial response signal.
 51. The apparatus of claim42, wherein the PR FIR filter has a target response of an equalizedpartial response signal in accordance with the equation of${Y(D)} = {\sum\limits_{k = 0}^{k = {l - 1}}{x_{k}D^{- k}}}$ where 1 isan order of the equalized partial response signal, D is a delay, andx_(k) are coefficients chosen to fit the constraints of the opticalcommunication channel.
 52. The apparatus of claim 51, wherein the orderof the equation Y(D) is two and the coefficients are set to one suchthat the equation Y(D) simplifies to (D+1) for the target response. 53.The apparatus of claim 51, wherein the order of the equation Y(D) isthree and the coefficients are set to one such that the equation Y(D)simplifies to (D²+D+1) for the target response.
 54. The apparatus ofclaim 42, further comprising: an automatic gain controller (AGC) coupledbetween the optical-to-electrical converter and the matched filter, theautomatic gain controller having an input coupled to the output of theoptical-to-electrical converter to receive the received spread-pulsesignal, the automatic gain controller further having an output coupledto an input of the matched filter, the automatic gain controller tomaintain an amplitude of the received spread-pulse signal within a rangeof predetermined amplitudes on the output of the automatic gaincontroller.
 55. The apparatus of claim 42, wherein the data encoder is arun length limited data encoder and the transmit data is encoded intothe coded data using a run length limited code, and the data decoder isa run length limited data decoder and the received encoded data signalis decoded into the received data using the run length limited code. 56.A receiver for an optical communication channel, the apparatuscomprising: an optical-to-electrical converter having an optical inputto receive light pulses from an optical communication channel, theoptical-to-electrical converter further having an output, theoptical-to-electrical converter to convert the received light pulsesfrom an optical signal into an electrical signal as a receivedspread-pulse signal at the output of the optical-to-electricalconverter; a matched filter coupled to the optical-to-electricalconverter to receive the received spread-pulse signal, the matchedfilter further having an output, the matched filter to filter thereceived spread-pulse signal to optimize a signal to noise ratio; apartial response (PR) finite impulse response (FIR) equalizing filterhaving an input coupled to the output of the matched filter, the PR FIRequalizing filter to equalize the received spread-pulse signal into apredetermined partial response signal; and a maximum likelihood sequenceestimation (MLSE) detector having an input coupled to an output of thePR FIR equalizing filter, the MLSE detector to equalize thepredetermined partial response signal and recover data bits transmittedover the optical communication channel.
 57. The receiver of claim 56,wherein the recovered data bits are encoded using a code, and thereceiver further includes a data decoder having an input coupled to anoutput of the MLSE detector to receive the recovered data bits, the datadecoder further having an output, the data decoder to decode therecovered data bits into received decoded data at the output of the datadecoder.
 58. The receiver of claim 56, further comprising: an automaticgain controller (AGC) coupled between the optical-to-electricalconverter and the matched filter, the automatic gain controller havingan input coupled to the output of the optical-to-electrical converter toreceive the received spread-pulse signal, the automatic gain controllerfurther having an output coupled to an input of the matched filter, theautomatic gain controller to maintain an amplitude of the receivedspread-pulse signal within a range of predetermined amplitudes on theoutput of the automatic gain controller.
 59. The receiver of claim 57,wherein the code is a run length limited code, and the data decoder is arun length limited data decoder and the recovered data bits are decodedinto the received decoded data using the run length limited code. 60.The receiver of claim 56, wherein the matched filter is an analog filterthat has a response substantially matched to a response of the pulsefilter.
 61. The receiver of claim 60, wherein the pulse filter and thematched filter are Bessel filters.
 62. The receiver of claim 56, whereinthe matched filter is a filter having a transfer function with a pulseresponse substantially in accordance with${A( {z,t} )} \approx {\frac{\overset{\sim}{A}( {0,\frac{t}{B_{2}z}} )}{\sqrt{2\pi\quad B_{2}z}}{\exp( {{- i}\frac{t^{2}}{2B_{2}z}} )}}$where T₀ ² is much less than B₂z.
 63. The receiver of claim 56, whereinthe MLSE detector is a Viterbi detector to equalize the predeterminedpartial response signal and recover the recovered data bits.
 64. Thereceiver of claim 56, wherein the MLSE detector removes intersymbolinterference (ISI) from the predetermined partial response signal andrecover the recovered data bits.
 65. The receiver of claim 56, whereinthe PR FIR equalizing filter has a target response of an equalizedpartial response signal in accordance with the equation of${Y(D)} = {\sum\limits_{k = 0}^{k = {l - 1}}{x_{k}D^{- k}}}$ where 1 isthe order of the equalized partial response signal, D is a delay, andx_(k) are coefficients chosen to fit the constraints of the opticalcommunication channel.
 66. The receiver of claim 65, wherein the orderof the equation Y(D) is two and the coefficients are set to one suchthat the equation Y(D) simplifies to (D+1) for the target response. 67.The receiver of claim 65, wherein the order of the equation Y(D) isthree and the coefficients are set to one such that the equation Y(D)simplifies to (D²+D+1) for the target response.
 68. A transmitter for anoptical communication channel, the transmitter comprising: a partialresponse precoder having an input to receive a transmit data signal, theprecoder further having an output, the precoder to correlate bits of thetransmit data together into a precoded signal at the output of theprecoder; a pulse filter having an input coupled to the output of theprecoder to receive the precoded signal, the pulse filter further havingan output, the pulse filter to spread out the pulses in the precodedsignal into a spread-pulse signal at the output of the pulse filter; andan electrical-to-optical converter having an input coupled to the outputof the pulse filter, the electrical-to-optical converter further havingan optical output to transmit light pulses over the opticalcommunication channel, the electrical-to-optical converter to convertthe spread-pulse signal from an electrical signal into the light pulsesat the optical output.
 69. The transmitter of claim 68, furthercomprising: a data encoder having an input to receive transmit data, thedata encoder further having an output coupled to the input of theprecoder, the data encoder to encode the transmit data into the transmitdata signal at the output of the data encoder.
 70. The transmitter ofclaim 69, wherein the data encoder is a run length limited data encoder,and the transmit data is encoded into the transmit data signal using arun length limited code.
 71. The transmitter of claim 68, wherein in atime domain the pulse filter spreads out the pulses of the precodedsignal into the spread-pulse signal in accordance with a raised cosinefilter response.
 72. The transmitter of claim 68, wherein the pulsefilter spreads out the pulses of the precoded signal into thespread-pulse signal in accordance with a Gaussian filter response.
 73. Atransceiver for an optical communication channel, the transmittercomprising: a transmitter including, a precoder having an input toreceive a transmit data signal, the precoder further having an output,the precoder to correlate bits of the transmit data together into aprecoded signal at the output of the precoder, a pulse filter having aninput coupled to the output of the precoder to receive the precodedsignal, the pulse filter further having an output, the pulse filter tospread out the pulses in the precoded signal into a spread-pulse signalat the output of the pulse filter, and an electrical-to-opticalconverter having an input coupled to the output of the pulse filter, theelectrical-to-optical converter further having an optical output totransmit a first plurality of light pulses over a first optical fiber,the electrical-to-optical converter to convert the spread-pulse signalfrom an electrical signal into the first plurality of light pulses atthe optical output; and, the receiver including, anoptical-to-electrical converter having an optical input to receive asecond plurality of light pulses from a second optical fiber, theoptical-to-electrical converter further having an output, theoptical-to-electrical converter to convert the second plurality of lightpulses from an optical signal into an electrical signal as a receivedspread-pulse signal at the output of the optical-to-electricalconverter, a matched filter coupled to the optical-to-electricalconverter to receive the received spread-pulse signal, the matchedfilter further having an output, the matched filter to filter thereceived spread-pulse signal to optimize a signal to noise ratio, apartial response (PR) finite impulse response (FIR) equalizing filterhaving an input coupled to the output of the matched filter, the PR FIRequalizing filter to equalize the received spread-pulse signal into apredetermined partial response signal, and a maximum likelihood sequenceestimation (MLSE) detector having an input coupled to an output of thePR FIR equalizing filter, the MLSE detector to equalize thepredetermined partial response signal and recover data bits of areceived data signal.
 74. The transceiver of claim 73, wherein thetransmitter further includes a data encoder having an input to receivetransmit data, the data encoder further having an output coupled to theinput of the precoder, the data encoder to encode the transmit data intothe transmit data signal at the output of the data encoder.
 75. Thetransceiver of claim 74, wherein the data encoder is a run lengthlimited data encoder, and the transmit data is encoded into the transmitdata signal using a run length limited code.
 76. The transceiver ofclaim 73, wherein the pulse filter of the transmitter spreads out thepulses of the precoded signal into the spread-pulse signal in accordancewith a raised cosine filter response.
 77. The transceiver of claim 73,wherein the pulse filter of the transmitter spreads out the pulses ofthe precoded signal into the spread-pulse signal in accordance with aGaussian filter response.
 78. The transceiver of claim 73, wherein thereceived data signal is encoded using a code, and the receiver furtherincludes a data decoder having an input coupled to an output of the MLSEdetector, the data decoder further having an output, the data decoder todecode the received data signal into decoded data bits at the output ofthe data decoder.
 79. The transceiver of claim 78, wherein the receiverfurther includes, an automatic gain controller (AGC) coupled between theoptical-to-electrical converter and the matched filter, the automaticgain controller having an input coupled to the output of theoptical-to-electrical converter to receive the received spread-pulsesignal, the automatic gain controller further having an output coupledto an input of the matched filter, the automatic gain controller tomaintain an amplitude of the received spread-pulse signal within a rangeof predetermined amplitudes on the output of the automatic gaincontroller.
 80. The transceiver of claim 78, wherein the code is a runlength limited code, and the data decoder is a run length limited datadecoder and the received data signal is decoded into the decoded databits using the run length limited code.
 81. The transceiver of claim 78,wherein the matched filter of the receiver is an analog filter that hasa response substantially matched to a response of the pulse filter. 82.The transceiver of claim 81, wherein the pulse filter and the matchedfilter are Bessel filters.
 83. The transceiver of claim 73, wherein thematched filter is a filter having a transfer function with a pulseresponse substantially in accordance with${A( {z,t} )} \approx {\frac{\overset{\sim}{A}( {0,\frac{t}{B_{2}z}} )}{\sqrt{2\pi\quad B_{2}z}}{\exp( {{- i}\frac{t^{2}}{2B_{2}z}} )}}$where T₀ ² is much less than B₂z.
 84. The transceiver of claim 78,wherein the MLSE detector of the receiver is a Viterbi detector toequalize the predetermined partial response signal and recover the databits of the received data signal.
 85. The transceiver of claim 78,wherein the MLSE detector of the receiver removes intersymbolinterference (ISI) from the partial response signal and recover the databits of the received data signal.
 86. The transceiver of claim 78,wherein the PR FIR equalizing filter of the receiver has a targetresponse of an equalized partial response signal in accordance with theequation of ${Y(D)} = {\sum\limits_{k = 0}^{k = {l - 1}}{x_{k}D^{- k}}}$where 1 is an order of the equalized partial response signal, D is adelay, and x_(k) are coefficients chosen to fit the constraints of theoptical communication channel.
 87. The transceiver of claim 86, whereinthe order of the equation Y(D) is two and the coefficients are set toone such that the equation Y(D) simplifies to (D+1) for the targetresponse.
 88. The transceiver of claim 86, wherein the order of theequation Y(D) is three and the coefficients are set to one such that theequation Y(D) simplifies to (D²+D+1) for the target response.